This invention relates in general to means for providing real time control of the amplitude of an electric wave and more specifically to means for providing real time expansion or compression of the amplitude of an electric wave which corresponds to a sound wave such as continuous discourse.
Wave expanders or compressors are well known and are generally called automatic gain control (a.g.c.) or automatic volume control (a.v.c.) circuits. Such known circuits generally store a portion of a wave whose volume is to be controlled over a short period of time and average the wave over a short period of time such as a millisecond. The average voltage so produced is used to control the volume of an electrical wave that comes along after the average voltage has been produced. Therefore, such known volume control devices are not real time devices. That is, they control the volume of an existent wave in accordance with the average amplitude of previously existing waves. Such known volume control devices therefore, may distort the electrical wave so that the listener to the sound to which the electrical wave is finally converted may not be able to understand the sound although he may have no trouble hearing it. For example, plosive sounds like the "t", "p" sounds may be compressed in accordance with the amplitude of a loud sound preceeding them, whereby the plosive sound may lose much of its plosive quality, thereby making these sounds hard to understand. Furthermore, soft sounds often follow louder sounds in normal speech and when the soft sound is compressed in accordance with the amplitude of an immediately preceeding loud sound, as is done in known compressors, the soft sound may be lost entirely. Therefore, where a known compressor is incorporated in a hearing aid for persons having impaired hearing, or where a known compressor is incorporated in a loudspeaker, which is also a hearing aid, for projecting sound into an area where there is background noise, the known sound compressor detracts from the readability of the sound. That is, due to the action of the known sound compressor, the sound may be heard but may not be intelligible to persons having impaired hearing or to persons who are in an area where there are background noises. In summary, while known compressors do compress the range of amplitude of sounds on which they operate, the compressors compress the amplitude range of all sounds as controlled by the stored wave. It has been found advantageous, as will be more specifically noted below, to compress or expand the amplitude of sounds without using the controlling action of previous stored amplitude, which cannot be done by presently known volume controls, to realize real time amplitude compression or expansion.